This invention relates to turbine seal technology, and more specifically, to active tip clearance control for shrouded gas turbine blades or buckets.
In the later stages of a gas turbine engine, it is common to provide a radially outer tip shroud connecting the tips of the blades or buckets in an annular row of such blades or buckets that are secured to axially spaced turbine wheels fixed to the turbine rotor. The top or radially outer edge of the shroud may be provided with one or more radially-projecting teeth to stiffen the shroud and to act as a labyrinth seal to reduce leakage of the working fluid over the shrouded buckets.
A clearance is necessary between the shroud tooth (or teeth) and the surrounding stator structure to prevent a rub during transient conditions (such as at start-up and shut-down or other significant load changes), but that clearance is to be reduced during normal operating conditions so as to minimize the leakage.
Sometimes the stator structure carries a honeycomb or other abradable surface which tolerates repeated rubs so a tighter clearance can be maintained. It is also known to use multiple teeth, some of which are carried on the tip shroud and others on the opposed stator surface. It is also known to move the stator surfaces radially inwardly to reduce the clearance once the turbine components have reached thermal equilibrium, while keeping large, safe clearances during starting and stopping.
There is also a clearance control system that involves shifting the rotor axially relative to the stator to adjust the gap between respective angled surfaces of shroudless buckets and similarly angled surfaces of the stator.